1. Technical Field
The present invention relates to a turbo compound engine system provided with a power turbine for recovering the exhaust gas energy and returning the recovered energy to the crankshaft of the engine. More particularly, the present invention relates to a turbo compound engine system which produces braking effort by allowing the power turbine to be rotated in a reverse sense by the crankshaft when a vehicle is decelerated.
2. Background Art
The Assignee of the present invention disclosed a "Turbo Compound Engine" in Japanese Patent Application No. 61-228107 which is a System for obtaining braking effort by rotating a power turbine in a reverse sense with the power turbine being provided in the exhaust gas line of the engine.
According to the above mentioned application, as depicted in FIG. 6, a power turbine 151 for recovering the exhaust gas energy is provided in the exhaust gas line 153, a fluid passage 155 which bypasses the power turbine 151 is connected to the exhaust gas line 153, and a fluid passage switching means 159 is provided to the fluid passage 155. The fluid passage switching means 159 is constructed such that the exhaust gas line 153a parallel to the fluid passage 155 may be closed when the exhaust brake is applied and rotation power is transmitted from the crankshaft 157 to the power turbine 151 while opening the fluid passage 155.
The power turbine 151 has to be driven at a rotational speed between 80,000 and 100,000 rpm in order to effectively exhibit its characteristics. Therefore, when the power turbine 151 is reversed in its rotational direction, a considerable load is imposed on a power transmission system 161 between the crankshaft 157 and the power turbine 151.
As indicated by braking effort characteristic curve I in FIG. 5, if the power turbine 151 functions as a compressor as the rotation of the power turbine 151 is reversed from its normal direction to its reverse direction, the load against the crankshaft 157 sharply increases, which is called overshoot, just after the rotation of the power turbine 151 reaches zero, i.e., just after the power turbine 151 starts rotating in a reverse sense. This is because the power turbine performs both compressor work and air mixing work when it is reversed. To overcome this shortcoming with the turbo compound engine of the above mentioned Japanese Patent application, the power transmission system 161 between the crankshaft 157 and the power turbine 151 has to possess enough strength to counterbalance the overshoot or the power turbine has to be rotated slow enough not to damage the power transmission system 161. Either case is not optimal in terms of total braking effort and cost.